1. Field of the Invention
This invention relates to a fluidizing gas control system in a fluidized-bed incinerator, and more particularly to a system for controlling the flow of fluidizing gas supplied to a fluidized-bed incinerator in which an object to be incinerated is ignited under formation of a fluidized bed of a fluid-forming medium.
2. Description of the Prior Art
A fluidized-bed incinerator is widely utilized for the incineration of an object such as sludge to be incinerated.
It is commonly known that such a fluidized-bed incinerator includes a wind box disposed in the bottom of the body of the incinerator and having connected thereto a supply line supplying oxygen-containing fluidizing gas, for example, air, and a combustion chamber called a freeboard disposed above the wind box in a relation partitioned from the wind box by a perforated member or plate. A granular fluid-forming medium such as cement clinker whose principal component is CaO is previously contained in a suitable quantity in the combustion chamber, and fluidizing gas from the supply line is forcedly supplied into the combustion chamber through the wind box and perforated member to form a fluidized bed of the fluid-forming medium in the combustion chamber. An object to be incinerated, for example, granular cakes formed by thoroughly mixing and kneading sludge and fine powdery coal are charged into the combustion chamber from above to be fluidized together with the fluid-forming medium and then ignited by the aid of auxiliary fuel injected from burners extending into the combustion chamber. Such a fluidized-bed incinerator is generally large in size, and it is generally difficult to determine the flow rate of the fluidizing gas supplied for forming the fluidized bed. A prior art practice for determining the flow rate of fluidizing gas has comprised fabricating a reduced-scale model of an actual fluidized-bed incinerator, experimentally operating such a model to plot a characteristic curve indicative of the relation between the velocity of fluidizing gas flowing through the fluidized bed and the pressure loss across the wind box and the combustion chamber, and estimating the optimum value of the flow rate of fluidizing gas on the basis of the characteristic curve thus obtained.
However, practical application of the flow rate of fluidizing gas thus determined experimentally on the model to the actual fluidized-bed incinerator is not necessarily successful for the efficient formation of the fluidized bed, and, in addition, costs including that required for the fabrication of the experimental model are considerably large resulting in want of economy.
Further, a problem to be also considered in such a fluidized-bed incinerator is that reduction of the quantity of nitrogen oxides, NO.sub.X, contained in exhaust gases produced during incineration is required from the aspect of prevention of environmental pollution. The quantity of this NO.sub.X is closely related with the concentration of residual oxygen in the exhaust gases, and, therefore, the quantity of NOx can be controlled by controlling the quantity of oxygen in the fluidizing gas which is the source of oxygen required for combustion. However, because of the fact that the optimum value of the flow rate of supplied fluidizing gas is primarily determined on the basis of the efficiency of formation of the fluidized bed as described above, it becomes necessary to control the quantity of oxygen while maintaining the optimum value of the flow rate of fluidizing gas.